This is not true. I detailed how the credits might be used in my eerier response. That said, UA refunds is also well-known for canned rejections.

That said, if you rebooked within the same ticket -- same confirmation number -- then there might still be a path to a refund if the fare rules include visa refusal, which some do. However, I couldn't find any UA fares listed from SGN to PLS, at least on my first attemp (I didn't check HAN, DAD or any other airport in Vietnam). Assuming you didn't save a copy of your fare rules -- which most people don't -- to look them up in ExpertFlyer, you'd need to post the re-tickeing date, the scheduled flights dates, and the route, including what airlines were operating each leg. When you say S/L, do you mean S on the way out and L on the way back, or were two flight legs in the same direction using different fare codes?

With that information, someone should be able to look up the rules and see if you have a case for a refund or if you're going to be asking for special treatment.